1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) system, and in particular to improvements in channel estimation and pilot tracking in this OFDM system.
2. Description of the Related Art
Wireless systems frequently operate under challenging conditions. These conditions include the complexity of the environment in which a communication channel is desired and the mobility of one or more users using the channel in such an environment. For example, objects in the environment can reflect a transmitted wireless signal. This phenomenon is called a multipath condition. To address multipath and other conditions, a wireless system can employ various techniques. One such technique is Orthogonal Frequency Division Multiplexing (OFDM).
In an OFDM system, a signal can be split into multiple narrowband channels (called sub-channels) at different frequencies. For example, current 802.11a OFDM systems include 52 sub-channels. Thus, a transmitted signal could be represented by x−26 . . . x−1, x1 . . . x26, wherein both negative and positive side frequencies are included. In this configuration, each sub-channel carries a portion of the signal. Each sub-channel is “orthogonal” (i.e. independent) from every other sub-channel. Multipath conditions and noise can result in deterioration of this orthogonality.
In an attempt to restore orthogonality, the 1999 IEEE 802.11a standard provides that a transmitted data packet includes a preamble, which precedes the actual data. FIG. 1 illustrates a portion of a data packet 100 including a preamble 105. As defined in the 802.11a standard, preamble 105 includes 10 “short” identical known symbols 101A-101J of length 16 (hereinafter shorts 101) concatenated to 2 “long” identical known symbols 102A-102B of length 64 (hereinafter longs 102). Note that a symbol refers to any waveform in time (e.g. represented as voltage versus time).
Longs 102 can be used to provide channel estimation. Specifically, because longs 102 are known, the receiver can use these symbols to provide rough channel estimations for a subsequent data symbol 103 in the data packet. In this manner, longs 102 can thereby increase the likelihood that the received data symbols can be correctly interpreted. Longs 102 are also called “training” symbols because they can “train” a frequency domain equalizer to learn about channel conditions.
Shorts 101 can be used to determine any frequency offset between the oscillators in the receiver and transmitter. Additionally, shorts 101 can be used to provide an initial system time synchronization. System time synchronization can also be continuously tuned using the data symbols, as now described in further detail.
The 802.11a standard provides that guard intervals (called GIs or mechanism cyclic prefixes) should be placed before longs and data. Specifically, a double guard interval (GI2) is placed before long symbols 102A-102B, thereby forming part of longs 102. In contrast, a regular guard interval (GI) is placed before data 103A, thereby forming part of data symbol 103. The double guard interval, as the name implies, is twice as long as the regular guard interval. In accordance with the 802.11a standard, the guard intervals are derived from their associated symbols. For example, FIG. 2 illustrates simplified data 202, which could be generated by 64 samples over time. A guard interval 201 essentially copies a last portion of data 202 and attaches it to the beginning of data 202. For example, in the 802.11a standard, the last ¼ of the symbol is copied. A last portion of longs 102 can also be copied to form double guard interval GI2 (e.g. in the 802.11a standard, essentially the last half of long symbol 102B).
A difference between the frequency of the transmitter and receiver oscillators can adversely and significantly impact system performance. For example, if the receiver's clock is not aligned with the incoming data, then sampling of the received signal could be sub-optimal. For this reason, “pilots”, also known signals (e.g. −1 and 1) defined by the 802.11a standard, are provided on 4 of the 52 orthogonal sub-channels to track and correct the difference between clocks.
For example, FIG. 3 illustrates a data symbol 300 including a GI 301 and data 302. If the receiver's clock samples earlier in time than the incoming data, then instead of detecting the values indicated by 1st sampling 303, the values indicated by 2nd sampling 304 could be detected. This de-synchronization can result in a phase ramp 400 in the frequency domain, as shown in FIG. 4. Note that phase ramp 400 is negative when the receiver's clock samples earlier in time than the incoming data and positive when the receiver's clock samples later in time than the incoming data. Because of the continual “slide” in sampling (see FIG. 3), the slope of the phase can continue to rotate symbol by symbol.
Moreover, when a signal is transmitted, the signal is modulated by the channel frequency, thereby improving its propagation properties in the channel. The modulation is based on the clock at the transmitter. Thus, at the receiver, the signal must be demodulated. This demodulation can result in some residual phase, which can be represented by an offset 401. The 4 pilots provided by the 802.11 standard attempt to track the phase slope and phase offset, thereby allowing the system to compensate for such slope and offset when necessary.
Unfortunately, sending just the two long symbols can be insufficient for accurate channel estimation, particularly when noise is present. Moreover, using four sub-channels can also be insufficient to compensate for phase slope and offset. Therefore, a need arises for a system and method for improving channel estimation and pilot tracking.